This invention relates to a magnetic recording medium, magnetic recording apparatus and magnetic recording method, and more particularly, to a magnetic recording medium, magnetic recording apparatus and magnetic recording method capable of higher-density recording than conventional recording by suppressing thermal fluctuation.
Along with recent progress of computers in processing speed, there is a continuous demand on higher speeds and higher densities also of hard disk drives (HDDs). Increase of the density, however, is physically limited, and this is an issue whether the techniques can continuously meet such request or not.
Magnetic recording mediums for substantially recording information have a magnetic layer comprising of minute magnetic grains. For high-density recording, magnetic domains recorded on the magnetic layer must be small enough. To enable clear distinction of small recording magnetic domains, the boundaries of the magnetic domains should be smooth enough, and this results in the reduction of the size of the magnetic grains. The propagation of magnetization reversal among the grains distorts the domain boundary. Therefore, individual magnetic grains need to be isolated magnetically by nonmagnetic substances to prevent exchange coupling interaction among the magnetic grains. Moreover, the magnetic layer must be thin enough for the high-density recording, from the viewpoint of the magnetic interaction between the magnetic head and the recording medium.
To satisfy those requirements, the unit volume of the magnetization reversal (whose size becomes almost equal to that of a magnetic grain as more and more of those requirements are met) must be decreased more and more as the density increases. However, if the magnetization reversal unit is diminished, then the magnetic anisotropy energy of the unit (Ku (density of magnetically anisotropy energy)×V (volume of the magnetization reversal unit)) becomes smaller than the thermal fluctuation energy, and it will be no longer possible to maintain the domains. This is the “thermal fluctuation phenomenon”, and the physical limit of recording density mainly governed by this phenomenon is called “thermal fluctuation limit”.
Magnetization reversal by thermal fluctuation can be prevented if the magnetic anisotropy energy density Ku is increased. However, since the recording coercivity Hcw is approximately proportional to Ku, magnetic recording is impossible even by the state-of-art recording head, which can generate the highest magnetic field in the world.
Increasing the volume V of the magnetization reversal unit is another way. However, increase of the size of magnetic grain along the plane of the medium is against the requirement of the high-density recording. In addition, when V is increased by increasing the thickness of the medium, the recording field from the head cannot reach a full-depth of the medium and fails to reverse the magnetization. Here again, high-density recording is not possible.
As discussed above, conventional magnetic recording mediums involve the problem that enhancement of the density encounters a limit caused by the thermal fluctuation phenomenon.